Organometallic compounds of Group I metals, such as lithium, are widely used to initiate the anionic polymerization of diolefin monomers into rubbery polymers. For instance, alkyl lithium compounds are widely used on a commercial basis to initiate polymerizations of conjugated diolefin monomers and copolymerizations of conjugated diolefin monomers with vinyl aromatic monomers. For example, n-butyl lithium can be employed to initiate the copolymerization of 1,3-butadiene monomer and styrene monomer in the synthesis of styrene-butadiene rubber.
The organometallic compounds which are used to initiate such polymerizations generally only contain one lithium atom per molecule. Alkyl lithium compounds having the formula Li--R, wherein R represents as alkyl group containing from 2 to 8 carbon atoms, are the most widely used initiators of this type. Their utilization normally results in the production of a polymer which is linear rather than branched. To improve polymer performance characteristics polymers made with such monolithium compounds are often branched or coupled by the addition of coupling agents, such as silicon tetrachloride or tin tetrachloride. Multifunctional compounds, such as divinyl benzene, are sometimes included as a polymerization ingredient to induce branching.
It is also known in the art that compounds which contain multiple lithium atoms can be used to initiate polymerizations and that the polymers produced by such polymerizations are branched. For instance, U.S. Pat. No. 5,262,213 discloses the use of organo multifunctional lithium compounds as polymerization initiators. U.S. Pat. No. 5,262,213 indicates that these multifunctional lithium compounds will typically be organodilithium compounds or organotrilithium compounds. These multifunctional organolithium compounds include as representative examples 1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene, 1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane, 1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane, 1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane, 1,2,4,6-tetralithiocyclohexane, 4,4'-dilithiobiphenyl.
The employment of organodilithium compounds as polymerization initiators does not result in the formation of branched polymers. However, organolithium compounds which contain three or more lithium atoms will normally be branched. The number of branch sites in the polymer produced will normally be equal to the number of lithium atoms in the organolithium compound minus two.